1. Field of the Invention
The present invention relates to an image display device, and more particularly to an image display device having a liquid crystal display panel in which display picture elements are arranged in dot matrix form and in which a frame is displayed around the periphery of the display portion of an inputted image signal in cases in which the number of display picture elements that display an inputted image signal are fewer than the number of picture elements in the display panel.
2. Description of the Related Art
A prior-art example of an image display device in which image picture elements are arranged in dot matrix form is disclosed in, for example, Japanese Patent Laid-open No. 160426/91. In this example of the prior art, each of the RGB colors in a projector system using a frontal projection tube are reconstituted on the surface of a reflecting screen to form a color image on the screen surface. In this case, a frame image is projected around the original image by fixing the inputted RGB signal levels at prescribed levels in the vertical blanking interval and horizontal blanking interval.
Japanese Patent Laid-open No. 89784/91 further discloses technology in this field. In this example of prior art, in cases in which the light intensity of a light source is modulated by a liquid crystal light bulb and projected onto a screen by way of projection optics to form an image, blanking interval processing is effected by monotonously varying the length of the horizontal blanking interval changed in proportion to the vertical blanking interval to avoid trapezoid distortion.
These examples of the prior art therefore differ from the object of the present invention, which is a method of producing a display frame in cases of input of an image signal having fewer image picture elements than the number of picture elements of the image display device in an image display device arranged in dot matrix form.
As examples of the prior art that are directly related to the present invention, cases will next be described in which various types of image signals are displayed on a liquid crystal display device constructed from a common liquid crystal panel using TFT (thin-film transistor) elements.
The liquid crystal panel shown in FIG. 1 is constructed as follows. Liquid crystal display elements that ordinarily employ TFT elements as switching elements of voltage applied to liquid crystal picture elements are of a construction in which the drain terminals of the TFT elements are connected to the liquid crystal picture element electrodes, and vertical and horizontal gate terminals and source terminals are connected in common. By applying voltage to the common gate terminals that brings about a conductive state between the source and drain of TFT elements, ON voltage is applied to all TFT elements (usually, one horizontal line portion of a display screen) connected to the common gate terminals, whereby the source-drain of the TFT elements enters a conductive state. Writing of image signals to liquid crystal picture elements is then carried out by sequentially applying the voltage of each respective picture element to the common source terminals as image signals.
The common gate terminals and common source terminals are driven by two types of drive circuits, referred to as source driver 104 and gate driver 105, which are connected to each terminal.
In a case in which input signals for picture elements numbering 1280 horizontally and 1024 vertically are displayed on liquid crystal panel 108 having picture elements for 1280 vertical dots by 1024 horizontal dots as shown in FIG. 1, writing of the frame is not necessary because the image data for one picture element can be displayed on each respective dot of the liquid crystal panel.
However, in the case of displaying image signals that have fewer picture elements than the number of picture elements of the image display device both for the number of horizontal and vertical picture elements, for example, if the number of picture elements of input signals is 640 horizontally and 400 vertically, the image data for one picture element may be displayed as four dots on the liquid crystal panel by doubling the image data for one picture element both horizontally and vertically, thereby realizing a display using as the display surface 1280 dots horizontally and 800 dots vertically of the picture elements of the liquid crystal panel, as shown in FIG. 2A. Here, a frame display is realized by applying either a black-level voltage or another voltage in the vertical blanking intervals for the picture elements of the areas 110 and 111 in which an image is not displayed in the vertical direction.
For a case in which the number of picture elements of the input signal that are thus doubled horizontally and vertically does not exceed the number of picture elements of the liquid crystal panel, the image may be displayed larger by doubling horizontally and vertically. For input signals that can be handled at other integer powers that do not exceed the number of picture elements of the employed liquid crystal panel either horizontally or vertically, display that is enlarged by an integer power may be carried out by multiplying by an integer.
In a case in which the number of picture elements of input signals is, for example, 800 horizontally and 600 vertically, all of the picture elements of the liquid crystal panel may be driven by displaying the input image signals in the center of the liquid crystal panel and displaying the peripheral portion as black, as shown in FIG. 2B.
In this case, the drive of picture elements in areas 112 and 113 in which an image is not displayed in the horizontal direction is effected in the horizontal blanking interval (FIG. 3A), and drive of picture elements of areas 110 and 111 in which an image is not displayed in the vertical direction is effected in the vertical blanking interval (FIG. 3B).
In some cases, the time of the blanking interval is inadequate for some types of input signals, and sufficient time is thus not available for writing to all picture elements corresponding to a processed frame portion at usual speeds for displaying images. In other words, if the number of picture elements of input signals within one horizontal interval (for example, the number of samplings when digitizing the image data for digital processing) falls short of the number of picture elements in the horizontal direction of the image display device on which the image is to be displayed, not all of the picture elements in the horizontal direction within one horizontal interval can be driven.
In the prior art, such cases could be handled by reducing the time for writing to picture elements, i.e., by somewhat accelerating the shift clock (CLX or CLY) of the source driver when there is not enough time for the horizontal direction or of the gate driver when there is not enough time for the vertical direction.
In methods in which the source driver is accelerated, however, there are cases in which sufficient time was not available for applying voltage to each picture element, with the resulting drawback that, in cases in which the frame around the periphery of a display of an input image is to be, for example, black, an adequate black-level signal could not be written to the relevant picture elements and a satisfactory display of a black frame could not be obtained.